I have a required top-level parameter in my stylesheet:
<xsl:param name="citation-style" required="yes" as="xs:string" />
Currently, if that's missing, I get this error:
Circular definition of variable citation-style
Transformation failed: Run-time errors were reported
... which is not helpful.
That suggests there's another error present which is masking this one. Would
need to see more of the stylesheet to find it.
You can write your own ErrorListener to intercept errors and change the
messages if you want - but it's probably a mug's game, since on the
occasions when an inappropriate error message is produced, it's usually
because there's insufficient information available internally, and your
ErrorListener is unlikely to be able to deduce the true cause when Saxon
gets it wrong.
Michael Kay
http://www.saxonica.com/